Specify the wrong Spring and your new stoplight switch could be ruined in no time. Too short and it will damage the switch by straining the mechanism, too long and it just won't work, or jam sideways damaging the switch itself!
Order your Spring from us, and it will be correct for the model type specified, so will work properly, without causing your stop switch any problems. Better still order the Spring and the new switch at the same time, if for no other reason that using an old part with a new part looks pretty awful!